Changes

core: guard TCP stats on availability of linux/tcp.h and struct tcp_info

Detect availability of linux/tcp.h and struct tcp_info at configure time
and build TCP statistics code conditionally based on that.

This replaces platform-specific conditionals with proper feature checks
and avoids build failures on systems lacking Linux TCP headers or types
(e.g. non-Linux or cross-build environments).

When TCP_INFO support is unavailable, the TCP stats code is compiled out,
while keeping the public API intact.

No functional changes on systems where linux/tcp.h and struct tcp_info
are available.

Change-Id: Ibcd00f15131b0291f0b10eca51401c518b77cc39